For many years, graphite composite sports racquet frames have been produced by manual labor in molds using air injection. In a typical process, a “layup” is created by manually rolling multiple sheets or laminations (which take the form of strips of planar material), commonly formed of fibrous material, such as carbon or graphite fiber or fiberglass, to form a bladder. The bladder is formed, for example, using a number of “sheets” of graphite fiber, permeated, for example saturated, with an uncured thermoplastic or thermosetting resin. Generally, the sheets are wrapped by hand around a rigid mandrel or rod to control the desired layup shape, which is usually in the shape of a tube.
Before the graphite is wound on the mandrel, the mandrel is wrapped with a layer of material meant to form the internal surface of a layup bladder to be inflated during the manufacturing process as set forth below. The sheets are made of carbon fibers and, as alluded to above, permeated with an uncured plastic resin. These carbon fiber/resin sheets are manually cut into strips or ribbons prior to their assembly into a layup, typically by being wound around a mandrel to form a tube. After being wound, the layup (which, after winding, takes the form of a bladder) is manually formed into a desired racquet shape, reinforced with additional patches of the planar strips of carbon fiber permeated with resinous material, and placed in a mold. By resinous material is meant any material which can be used in graphite or fiberglass composite to bind the fibers into a substantially rigid structure.
The mold is then manually closed. The bladder is subjected to heat by the mold, causing the thermoplastic resin to cure. The result in the finished racquet is a hardening of the material of the layup. Alternatively, thermosetting resins may be used.
The bladder is then inflated with a manually placed single air nozzle, which is individually attached to one end of the bladder. The nozzle feeds, for example, air pump generated compressed air to force the walls of the layup to the interior walls of the mold cavity. See Hsu, U.S. Pat. No. 4,511,523 (1985). Because both ends of the layup are formed with an open configuration, the terminal end of the layup opposite the end being inflated by the nozzle is coupled, for example, to a capping or other sealing structure or artifice, allowing the buildup of pressure in the layup. The mold is then heated to cure the thermoplastic resin.
An artifact of this process is that composite racquet frames are commonly of a single-tube design. The two ends of the layup tube may be, for example in the case of tennis racquets, at the bottom of the racquet. In other words, the tube begins at the base of the tennis racquet handle, proceeds in a substantially straight direction along the length of the tennis racquet handle, extends around the oval string supporting frame shape and continues contiguous with the initial portion of the tube in a substantially straight direction back to the base of the tennis racquet handle. Typically, the base of the handle is cut at its end with a saw and a pair of gripping members secured around those portions of the tube ends which define the tennis racquet handle. Because of the requirement of air injection, the tubular basic shape is required to blow air through the entire tube which means the immediate output product is hollow with the above technique and necessarily has an open ended shaft at the bottom or base of the tennis racquet handle.
While it may be possible, in principle to the form a multiple-tube composite structure, it has been found that any internal divisions, bridges or lumens placed in these tubes are difficult to control in their placement because of variations in bladder air pressure and layup characteristics along the length of the layup. Attempts to include them in the past have been found to cause significant quality control and production problems.
On the one hand, due to the hollow nature of graphite racquets, a minimum cross-sectional width is required in, for example, the hoop or head portion of the tennis racquet in order to enable the tennis racquet to be strong enough to withstand the powerful strokes typical of the game. For example, professional tennis players can achieve ball speeds of 150 mph during a serve.
On the other hand, thinner frames are desirable because of the increase in swing speed, head speed, control and feel. The quality of the material which forms the racquet frame manufactured using the current state-of-the-art the air pressure process is such that the resulting hollow frame is limited to a minimum width, typically about 19 mm.